Be prepared. I suggest that you take a box, a way to water them, some food, and a heat source. If you have some way to heat water, a soda bottle filled filled with hot water and wrapped in a towel, or a sock full of rice heated up. I use a glucose drip bag, which works extremely well if you happen to have a source where you can get one.
They would be fine for 3 hours with just normal heat from the car heater. The post office doesn't treat them nearly as nice and they are sometimes days going through the postal service. Food, water, etc is only going to be useful if you break down and it takes longer to get them home.
While I would be able to offer water, I would not leave water in with them. If you put a heat source in with them make sure it can't roll or slide and hit them.